My 1996 Honda Civic hatchback idles too high at startup in cold conditions. Once the engine is warm the idle is fine, and in the summer there is no problem at all. What is a likely source for such sypmtoms?

 

How high? 1500-1600rpms is normal on cold engine start.

 

Well over 2,000 rpm. I don't have a tachometer but it is obviously way over a normal cold start high idle.

 

Get an external tach or an obd2 scanner to read real time rpm output.
check for vac leak. Best way is to puml smoke into the intake manifold via brake booster port off the manifold.
remove intake tube and seal the throttle body. See if smokes leak out anywhere.

 

Possibilties are:
Remove and clean IACV and throttle body. Clean and tighten (ring)FIT valve too if it has one.
Faulty coolant ECT (located at cylinder head under distributor). To test: https://honda-tech.com/forums/honda-...-unit-2982519/
Air in coolant system..may need to bleed.
Thermostat may be stuck open.
Vacuum leak that seals when engine is at operating temp (one suspect is the throttle body gasket)
Cracked cylinder head that seals at operating temp. If not losing coolant than not a concern. Also, more of a lumpy idle due to coolant filling only one cylinder (usually).
Maybe more than what I listed

 

There are a lot of places to check. I've been after this on my 89 Si for years and years. Still haven't found the source. If I let the clutch out, the car will even accelerate to 2k rpm. It's like the ECU wants it to be that high.

Do a vacuum leak test (with propane or carb cleaner) checking all possible leaks. Even where the master cylinder meets the brake booster, as well as where the throttle shaft passes through the throttle body (throttle spring and TPS sensor).

I even tried replacing the IACV, ECT, MAP & IAT with no effect (MAP & IAT replaced for a different reason though). I rebuilt the entire top end of the engine and intake manifold and it had no effect. Literally the only thing I have left to test is the throttle shaft bushings.